Each photographic printing process entails the coating of the substrate surface with photo-resist, which is usually of the negative type, exposing the surface through an interposed mask, and developing the exposed surface. The developed surface may be subjected to a number of different further treatments, such as etching, diffusion and aluminising. By control and selection of the mask patterns and processes a complex structure of layers may be built up. It is essential that the patterns reproduced on the surface at each exposure are in precise spatial relation to each other, and for this reason the mask used in each exposure of the surface must be precisely aligned with respect to previously produced patterns on the surface. It is for the purpose of achieving such precise alignment that fiducial marks are printed upon the surface and on the masks.
In one known method of aligning fiducial marks used hitherto, two fiducial marks are first printed upon the surface of a substrate such as a wafer which is to receive a complex structure of layers. The two fiducial marks are separated by a pre-determined distance which is usually comparable to the width of the wafer so that alignment to these marks necessarily causes alignment over the wafer as a whole. Each mark is basically in the form of a simple cross and the two marks are parallel to each other. Further fiducial marks, in the form of two crosses, separated by the same distance as those on the wafer, are provided on each of the masks with which the wafer is to be successively aligned. Alignment of the two wafer marks and the corresponding mask marks is achieved by automatically positioning the crosses of the wafer relative to those of each mask within the dimensions of uniform or blank regions surrounding the crosses using mechanical means and then using servomotors to effect further automatic positioning of the mask so that is fiducial crosses coincide with those of the wafer. Error signals to drive the servomotors are derived photoelectrically from optical scans of corresponding lines of respective mask and wafer crosses, a zero signal being provided at coincidence of the mask and wafer crosses. The regions surrounding the fiducial marks must be uniformly blank for automatic alignment since signals resulting from the optical scan can only be interpreted when no appreciable interfering mark falls within the scan.
After automatic alignment with the mask the wafer is exposed photographically to reproduce on the wafer the pattern of the mask. A problem which has existed hitherto is that at each exposure a further printing of the fiducial marks of the mask, such as the cross referred to above, is superimposed on the original mark on the wafer until several photographic images are superimposed on the wafer marks. The superimposed photographic prints inevitably give rise to spurious variations in the definition of the fiducial marks as regards width, shading and general contrast level, which result in some inaccuracy in the zero indication of the scanner. This inaccuracy increases as the number of superimpositions increases.
A known method of overcoming this problem is to use a different set of fiducial marks for each exposure. Each mask then reproduces photographically its particular set of fiducial marks, so that previous overprinted wafer fiducial marks are not used. The wafer itself is provided with fiducial marks unaffected by overprinting to permit accurate alignment of each successive mask. The set of wafer fiducial marks used corresponds to the set particular to each mask. Unfortunately, as has been explained above, for the purpose of automatic alignment there must necessarily be an isolating blank region around each fiducial mark to make photo-electric recognition feasible, and with multiple sets of wafer marks this entails a consequential loss in the effective usable area of the wafer. The lost area depends on the number of masking processes used and the expected variation in the initial mechanical positioning. This known method is applicable to manual alignment where recognition of close-spaced patterns through a microscope is feasible.